It varies depending on the drugs used, I've been under multiple times now, the big one being for an open heart bypass.
That one I saw nothing, felt nothing, but coming out of it I remember them pulling out the breathing tube and putting me on a bi-pap machine. I had to beg to be taken off of it because it was stopping me from exhaling. I could breathe in fine, but the back pressure wouldn't let me breathe out.
Then the drugs, it was a combination of a bunch of things, propofol (the stuff that killed Michael Jackson), fentanyl (the stuff that killed Prince). Oxy, the works.
I was having weird hallucinations. If I closed my eyes, I could see a perfectly painted brick wall about a foot in front of my face. I could see the detail on the bricks and the mortar, the texture of the paint. Bonus - every time they put me in a different room, the wall would change color.
No memory expansion = no sale. Clearly they had room for a MicroSD slot.